there isn't a specific field just for one calculation.  All calculations use the same field name, and if you are already including other calculations on the report, you likely just need to add the new calc # to your filter or criteria.  Unless it is a separate column, then you have to make a new calculated field for that new calculation.  Look at how the others ones are setup

To add on to what Char said, you could add a filter criteria for calculation# and specify the record number (from 5-3-1) of that calculation.